const double var_x[5] = {0.000, 1.000, 2.000, 3.000, 4.000};
const double var_y[5] = {1.951, 1.995, 3.088, 4.220, 6.553};
const double cov_y[5][5] = {
    {1.000, 0.020, 0.000, 0.000, 0.000},
    {0.020, 0.485, 0.103, 0.000, 0.000},
    {0.000, 0.103, 0.740, 0.170, 0.000},
    {0.000, 0.000, 0.170, 0.316, 0.214},
    {0.000, 0.000, 0.000, 0.214, 0.520}
};

double model(double x, double a, double b)
{
    return a*x + b;
}

double chisq(double a, double b)
{
    TVectorD vec(5);
    for (int r=0; r<5; r++) {
        double func = model(var_x[r],a,b);
        vec(r) = func-var_y[r];
    }
    TMatrixD cov(5,5);
    for (int r=0; r<5; r++) {
        for(int c=0;c<5;c++) {
            cov(r,c) = cov_y[r][c];
        }
    }
    return cov.Invert()*vec*vec;
}


void example_01()
{
    TH2D *Q2 = new TH2D("Q2","chisqure(a,b)",20,0.,2.,20,0.,2.);
    for (int j=0; j<20; j++) {
        for (int i=0; i<20; i++) {
            double a = (double)i*0.1+0.05;
            double b = (double)j*0.1+0.05;
            Q2->SetBinContent(i+1,j+1,chisq(a,b));
        }
    }
    Q2->GetXaxis()->SetTitle("a");
    Q2->GetYaxis()->SetTitle("b");
    Q2->SetStats(false);
    Q2->SetContour(64);
    Q2->Draw("colz");
}